1. Field of the Invention
The present invention relates generally to a semiconductor chip assembly, and more particularly to a semiconductor chip assembly in which a semiconductor chip is mechanically and electrically connected to a support circuit.
2. Description of the Related Art
Semiconductor chips have input/output pads that must be connected to external circuitry in order to function as part of an electronic system. The connection media is typically an array of metallic leads (e.g., a lead frame) or a support circuit (e.g., a substrate), although the connection can be made directly to a circuit panel (e.g., a mother board). Several connection techniques are widely used. These include wire bonding, tape automated bonding (TAB) and flip-chip bonding.
Wire bonding is by far the most common and economical connection technique. In this approach, wires are bonded, one at a time, from the chip to external circuitry by thermocompression, thermosonic or ultrasonic processes. In thermocompression bonding, fine gold wire is fed from a spool through a clamp and a capillary. A thermal source is swept past an end of the wire to form a wire ball that protrudes from the capillary. The chip or capillary is then heated to about 200 to 300xc2x0 C., the capillary is brought down over an aluminum pad, the capillary exerts pressure on the wire ball, and the wire ball forms a ball bond on the pad. The capillary is then raised and moved to a terminal on the support circuit, the capillary is brought down again, and the combination of force and temperature forms a wedge bond between the wire and the terminal. Thus, the connection between the pad and the terminal includes the ball bond (which only contacts the pad), the wedge bond (which only contacts the terminal) and the wire between the bonds. After raising the capillary again, the wire is ripped from the wedge bond, the thermal source is swept past the wire to form a new wire ball, and the process is repeated for other pads on the chip. Thermosonic bonding is similar to thermocompression bonding but adds ultrasonic vibration as the ball and wedge bonds are formed so that less heat is necessary. Ultrasonic bonding uses aluminum wire to form wedge bonds without applying heat. There are many variations on these basic methods.
TAB involves bonding gold-bumped pads on the chip to external circuitry on a polymer tape using thermocompression bonding. TAB requires mechanical force such as pressure or a burst of ultrasonic vibration and elevated temperature to accomplish metallurgical welding between the wires or bumps and the designated surface.
Flip-chip bonding involves providing pre-formed solder bumps on the pads, flipping the chip so that the pads face down and are aligned with and contact matching bond sites, and melting the solder bumps to wet the pads and the bond sites. After the solder reflows it is cooled down and solidified to form solder joints between the pads and the bond sites. Organic conductive adhesive bumps with conductive filler particles in polymer binders have been used in place of solder. A major advantage of flip-chip bonding over wiring bonding and TAB is that it provides shorter connection paths between the chip and the external circuitry, and therefore has better electrical characteristics such as less inductive noise, cross-talk, propagation delay and waveform distortion. In addition, flip-chip bonding requires minimal mounting area and weight which results in overall cost saving since no extra packaging and less circuit board space are used.
While flip-chip technology has tremendous advantages over wire bonding and TAB, its cost and technical limitations are significant. For instance, the cost of forming bumps on the pads is significant. As a result, bumpless techniques have been developed that combine wire bonding with a flip-chip arrangement. Several of these techniques are described below.
U.S. Pat. No. 4,442,967 discloses a method of providing a raised contact portion on a microcircuit. A wire ball is formed on a wire end by applying thermal energy, the wire ball is pressed against a contact area on the microcircuit using thermocompression or thermosonic wire bonding to form a ball bond, a weakened area of the wire is created near the ball bond, and the wire is severed at the weakened area to provide a raised contact portion on the contact area. The contact portions thus obtained are relatively simple and economical compared to electroplated bumps. Thereafter, the contact portions are electrically connected to a support circuit by disposing a conductive adhesive between the contact portions and bond sites in a flip-chip arrangement.
U.S. Pat. No. 4,661,192 discloses a method of bonding chips to support frames by providing ball bonds on chip pads using wire bonding, planarizing the ball bonds, coating the planarized ball bonds with conductive epoxy, and then registering and bonding the conductive epoxy to corresponding conductive patterns on support frames. Care must be taken to ensure that the epoxy does not flow excessively and cause shorting between the leads.
U.S. Pat. No. 4,955,523 discloses a technique for interconnecting electronic components in which interconnection wires are wire bonded to contacts on an integrated circuit chip, an insulating material is applied to the surface of the chip to encapsulate the bonds between the wires, the wires are aligned with solder pools located in recesses of an IC chip carrier, heat is applied to fuse the solder, and the wires are inserted into the fused solder.
U.S. Pat. No. 5,764,486 discloses interconnecting a flip-chip integrated circuit to a substrate using wire bumps. The wire bumps formed on the chip include a pointed tip, a stub portion and a ball portion. The pointed tips are brought in ohmic contact with conductors on the substrate. Thereafter, an adhesive material is disposed between the substrate and the chip and cured using ultraviolet light. Alternatively, a spring system provides compressive force between the substrate and the chip.
U.S. Pat. No. 5,813,115 discloses a method of mounting a semiconductor chip to a wiring substrate using conductive adhesive or solder. The method includes feeding a wire through a capillary, fusing an end of the wire to form a ball, pressing and bonding the ball against a chip pad, cutting the wire so that a remaining portion protrudes above the ball, pressing the protruding contact against a shaping platform coated with conductive adhesive to adjust the height of the protruding contact and transfer conductive adhesive onto the protruding contact, and bonding the protruding contact to a conductor on the wiring substrate using the transferred conductive adhesive. Alternatively, the protruding contact is pressed against a solder coated conductor on the wiring substrate and the solder is heated to bond the protruding contact to the conductor.
U.S. Pat. No. 5,863,816 discloses bonding conductive wires on bonding pads of a semiconductor chip, placing the structure in an electrolytic solution such that free ends of the conductive wires opposite the pads are exposed outside the solution, attaching a conductive plate to the free ends of the wires, and electroplating a surface layer on the wires except for the free ends. This strengthens the wires while permitting the free ends to contact solder joints that are subsequently formed between the wires and a printed circuit board.
A major drawback of these techniques is that solder joints or conductive adhesives connect the wires to the substrates.
Solder joints exhibit increased electrical resistance as well as cracks and voids over time due to fatigue from thermo-mechanical stresses. In addition, the solder is typically a tin-lead alloy and lead-based materials are becoming far less popular due to environmental concerns over disposing of toxic materials and leaching of toxic materials into ground water supplies.
Conductive adhesives do not normally form a metallurgical interface in the classical sense. Rather, the electrically conductive path is through conductive filler particles that contact one another. Moisture penetration through the polymer binder may induce corrosion or oxidation of the conductive filler particles resulting in an unstable electrical connection. Furthermore, the polymer binder and the conductive filler particles may degrade leading to an unstable electrical connection. Thus, conductive adhesives may have adequate mechanical strength but poor electrical characteristics.
Recent introduction of grid array packaging (e.g., ball grid arrays), chip size packages (CSP) and flip-chip packages using high density interconnect substrates are relentlessly driving increased printed circuit board density and increasing the demands upon semiconductor chip assemblies.
In view of the various development stages and limitations in currently available semiconductor chip assemblies, there is a need for a semiconductor chip assembly that is cost-effective, reliable, manufacturable, provides excellent mechanical and electrical performance, and complies with stringent environmental standards.
An object of the present invention is to provide a semiconductor chip assembly with a chip and a support circuit that provides a low cost, high performance, high reliability package.
Another objective of the present invention is to provide a convenient, cost-effective method for manufacturing semiconductor chip assemblies as chip size packages, ball grid arrays or other structures.
The present invention accomplishes these objectives by providing an elongated wire that is wire bonded to the chip and plated to the support circuit.
In accordance with one aspect of the invention, a chip assembly includes a semiconductor chip attached to a support circuit. The chip includes a conductive pad and the support circuit includes a conductive trace. An elongated wire that electrically connects the pad to the trace is attached to the pad by a wire bond and attached to the trace by a plated connection joint.
Preferably, an electrically conductive path between and in contact with the pad and the trace includes the elongated wire and excludes solder and conductive adhesive. It is also preferred that the elongated wire is compressible in a direction normal to major surfaces of the chip and the support circuit, thereby providing vertical compliance between the chip and the support circuit.
In accordance with another aspect of the invention, a method of manufacturing the semiconductor chip assembly includes attaching the elongated wire to the pad with a ball bond formed by thermocompression or thermosonic wire bonding, positioning the elongated wire proximate to the trace, and then attaching the elongated wire to the trace with a connection joint formed by electroless plating.
The method may include inserting the elongated wire into a blind via in the support circuit and using the elongated wire to vertically support the chip after forming the ball bond and before forming the connection joint. Alternatively, the method may include inserting the elongated wire into a through-hole in the support circuit and using stand-offs to vertically support the chip after forming the ball bond and before forming the connection joint.
The method may also include underfilling an insulative adhesive between and in contact with the chip and the support circuit after forming the connection joint such that the adhesive surrounds the elongated wire and contacts the connection joint.
An advantage of the present invention is that the semiconductor chip assembly can be a flip-chip arrangement that need not include TAB leads, solder joints or conductive adhesives. Another advantage is that the assembly can be manufactured using conventional wire bonding equipment. Still another advantage is that the assembly can be manufactured using low temperature processes which reduces stress and improves reliability. A further advantage is that the assembly can be manufactured using well-controlled wet chemical processes which can be easily implemented by circuit board, lead frame and tape manufacturers. Still another advantage is that the assembly can be manufactured using materials that are compatible with copper chip and lead-free environmental requirements.
These and other objects, features and advantages of the invention will be further described and more readily apparent from a review of the detailed description of the preferred embodiments which follows.